1. Field of the Invention
The present invention relates to a process for producing an alkylene carbonate. More particularly, the present invention is concerned with a process for producing an alkylene carbonate, which comprises reacting an alkylene oxide with carbon dioxide in the presence of a catalyst in a reactor to obtain a reaction mixture containing an alkylene carbonate, the reactor communicating with a process-side channel of a heat exchanger through a piping to form a circulation circuit, wherein the process is characterized in that it further comprises, during or after the reaction, flowing a process liquid through the circulation circuit including the reactor and the process-side channel of the heat exchanger while flowing a heat exchange medium having a temperature of from 140° C. to 200° C. through the heat exchange-side channel of the heat exchanger, thereby maintaining the inner temperature of the process-side channel at a level within the range of from 135° C. to 200° C. By the use of the process of the present invention, it becomes possible to prevent foulings from occurring on the inner wall of the process-side channel of the heat exchanger or wash-away fouls having occurred and adhered to the inner wall of the process-side channel of the heat exchanger. Therefore, the heat removing ability of the heat exchanger is not lowered, thereby enabling the stable, continuous production of an alkylene carbonate for a long period of time.
2. Prior Art
The reaction for producing an alkylene carbonate by reacting an alkylene oxide with carbon dioxide in the presence of a catalyst is a liquid phase reaction, and it generates a heat of reaction in a quantity as large as 96.23 kJ/mol. Therefore, in the commercial scale production of an alkylene carbonate, stable removal of the heat of reaction is especially important for the stable operation and safety of the production process. Generally, in the production of an alkylene carbonate, a heat exchanger is used for removing the heat of reaction. However, when the operation of the production system for producing an alkylene carbonate by reacting an alkylene oxide with carbon dioxide in the presence of a catalyst is continued for a certain period of time, foulings occur on the inner wall of the process-side channel of the heat exchanger, thereby lowering the heat removing ability of the heat exchanger. Due to such lowering of the heat removing ability of the heat exchanger, the conventional process has a problem in that it is difficult to produce an alkylene carbonate stably in a continuous manner.
For example, in the working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 50-14632, a double-wall heat exchanger or a multi-tube heat exchanger is used to remove the heat of reaction generated during the production of an alkylene carbonate. The production system used in this patent document comprises first to third reactors, and the production system is operated under conditions such that the reaction temperature of the first reactor is 175° C. and the reaction temperature of each of the second and third reactors is 180° C. However, this patent document has no description about a cooling medium fed to the heat exchanger and the temperature thereof. Further, a method for preventing foulings from occurring in the heat exchanger is not described in this patent document.
In the working examples of Unexamined Japanese Patent Application Laid-Open Specification No. Sho 54-98765, an alkylene carbonate was produced using a circulation circuit comprising a reactor and a heat exchanger. Specifically, for controlling the reaction temperature so that the temperature measured at the outlet of the reactor becomes 190° C., this patent document describes that water having a temperature of 110° C. was fed as a cooling medium to the cooling-side channel of the heat exchanger and that, as a result, steam (heat-exchanged cooling medium) discharged from the heat exchanger had a temperature of 160° C. or 170° C. However, this patent document also has no description about a method to prevent foulings from occurring in the heat exchanger.
In addition, although each of the above-mentioned patent documents describes a method for removing the heat of reaction generated during the production of an alkylene carbonate by means of a heat exchanger, the documents have no description about a chemical washing method for easily washing-away fouls having adhered to the inner wall of the process-side channel of the heat exchanger.
As conventional methods for cleaning and washing a heat exchanger, there can be mentioned mechanical washing methods in which fouls having adhered to the inner wall of the process-side channel of the heat exchanger are removed therefrom after disassembling the heat exchanger into parts thereof; and chemical washing methods in which fouls having adhered to the inner wall of the process-side channel of the heat exchanger are dissolved or stripped off with a chemical reagent without disassembling the heat exchanger. Specific examples of mechanical washing methods include a method in which the inner wall of a tube of the heat exchanger is washed using a brush, drill or the like which is driven by an electric power or an air motor; a method in which water is pressurized to 20 to 30 MPa using a high pressure pump and sprayed therethrough a small nozzle to thereby forcibly strip-off and wash-away the fouls from the heat exchanger; and a sand blast method in which sand (or iron particles) is sprayed onto the fouls by using highly pressurized air to thereby blow-off the fouls from the heat exchanger. For performing the chemical washing method, it is necessary to choose an appropriate material and conditions, e.g., the type and amount of a chemical reagent used as a washing liquid, a washing temperature and a washing method, depending on the type of fouls adhering to a heat exchanger (see, for example, pp. 1305–1306 of Akiyoshi Tamaoki ed., “Kagaku Puranto Kensetsu Binran (Handbook for Building Chemical Plants)” published by Maruzen Co., Ltd., Japan on Nov. 30, 1980, 7th printing). With respect to the washing of a heat exchanger which has been used for producing an alkylene carbonate, only mechanical washing methods have been practically used and, heretofore, neither an effective chemical reagent nor an effective chemical washing method is known in the art.